Cases of unexplained fetal and infantile death, including the so- called "sudden infant death syndrome" (SIDS) will be studied immunologically for evidence of active viral infection and for evidence of classical allergy, along with suitable control cases. These studies will be performed using intersecting pools of viral antigens and environmental allergens to facilitate the rapid screening of specimens for large numbers of different antibodies simultaneously. Highly purified (from specific immunosorbents) class specific antihuman immunoglobulin reagents labeled with I125 will be used to determine the immunoglobulin class of any antibodies discovered. Cases will be investigated for both systemic (serum) antibodies and local (respiratory tract) antibodies. Attempts will be made to isolate the appropriate virus from frozen material from cases with serological evidence of active or recent viral infection.